METAZOA PORIFERA. 69 



It may be that even pores do not exist, and if so the 

 water may be taken in and thrown out at the large open- 

 ing, but in the absence of any special apparatus for sift- 

 ing the water it is more reasonable to suppose that if this 

 body is really a sponge, as claimed by Haeckel, it would 

 when living and feeding in its native element have tem- 

 porary pores of minute size capable of opening through 

 the walls between the cells. These could furnish the 

 internal cavity with food of sufficiently minute size to be 

 handled by the flagella and to be swallowed by the micro- 

 scopic cells of the walls. 



Ascetta primordialis Hkl., is the simplest form now 

 known with certainty to be a sponge. If it were deprived 

 of its skeleton it would represent the simplest sponge 

 type, to which Haeckel 1 has given the name of Olynthus. 



The fertilized egg (PI. 61, figs. 1-3) of this species of 

 Ascetta undergoes segmentation and a one layered bias- 

 tula results. While still within the body of the parent 

 cells migrate from the surface of the blastula to its 

 interior central cavity and this process continues after the 

 larva has passed into the water (PI. 61, fig. 4) until the 

 cavity is filled (fig. 5). The adult Ascetta (PI. 62, fig. i ; 

 fig. 2, the same with a portion of the external wall 

 removed) is a simple bag which is capable of varying its 

 form so that at times it resembles a vase, a cylinder, a 

 pear, or even an egg. At one end it is attached, and 

 at the other there is a large opening. The walls of the 

 bag are thin and are pierced by numerous transient pores 

 which are supposed to open anywhere through the walls 

 of the body, not having any constant location. There 

 are no persistent canals but the water passes through the 

 shifting pores into the body cavity which is lined with 

 flagellate and collared endodermal cells. The middle 

 layer of cells known as the mesoderm is thin but gives 

 rise to one, three, and four rayed spicules which are 



!Rep. Chall. Exp., Zool., XXXII, part 82, 1889. 



